The present invention relates to optical devices. In particular, the present invention is related to switching devices, signal-processing devices, and logic implemented using photonic optical devices. More particularly, the present invention is related to a new class of optical devices, operated on the principle of transfer photon resistance, that are capable of performing multiple functions on signals carried by lightwaves or photons, including all-optical and electro-optical switching. The broad functionalities of these devices are similar to that of electronic transistors, except that electronic transistors operate on signals carried by RF current or electrons while the devices of the current invention operate on signals carried by lightwave or photons.
The current generation of computers utilizes a plurality of electronic transistor components. These transistors modulate the resistance to the motion of electrons (and thus current) in order to accomplish a wide variety of switching, amplification, and signal processing functions. Transistor electronic action controls or affects the motion of a stream of electrons through xe2x80x9ctransfer (electron) resistancexe2x80x9d via the action of another stream of electrons.
Electronic transistors are typically fabricated using semiconductors such as Silicon (Si), and to a far less extent Gallium Arsenide (GaAs). Computing functions are performed by such electronic transistors integrated or grouped together as logic circuits on a very large scale with high device density. Due to various reasons discussed below, however, electronic transistor computing in present implementations is ultimately limited to the maximum data clock speeds of a few GHz. Semiconductor electronic switches generally are thought to have theoretical upper limits on their performance. Achievable minimum switching times are thought to be in the tens of picoseconds (10-20 ps), while minimum achievable switching power consumption and operational energy are thought to be around 1 microwatt (1 xcexcW) and tens of femto-joule (10-20 fJ) levels, respectively. Such levels imply that high frequencies of operation may be possible for electronic computing.
Dense, high-frequency electronic circuit operations utilizing such electronic transistors present several persistent problems and complexities that, whether surmountable or not, are issues of concern to circuit designers. Even though electronic transistors that can operate at faster than tens of GHz do exist, the problems of electromagnetic interference (or xe2x80x9ccrosstalkxe2x80x9d), radiation, and parasitic capacitance in dense circuits limit the clock speed of electronic computers to a range of a few GHz as the signal wavelength through the circuit becomes comparable to the circuit size. Furthermore, high-frequency electronic circuits can suffer seriously from the problems of electromagnetic interference and radiation.
It is thought that an optical circuit for which the signals are carried by light instead of electrical current may be used to eliminate the problems involving electromagnetic interference. In order for an optical circuit to perform useful computational or signal processing functions, however, there must be a way to switch optical signals using other optical signals or electrical signals. The former case is referred to as xe2x80x9call-optical switchingxe2x80x9d and the latter case as xe2x80x9celectro-optical switchingxe2x80x9d.
Presently, fiber-optic communication systems typically use electro-optical switching. These optical communications systems have significant advantages over electrical communications systems utilizing electronic or radio-frequency (xcx9c109 Hertz) circuitries, partly because of the high frequency of light (xcx9c1014 to 1015 Hertz), which allows much broader bandwidths to be used to transmit signals. However, current electro-optical switches are large in size (usually at centimeter sizes or larger) and expensive. This makes it difficult to bring the high bandwidth fiber communication systems directly to the customer""s location, an undertaking which will require low-cost components capable of complex electro-optical signal processing. Thus, low-cost electro-optical devices and circuits capable of high-density of integration would be desirable. Besides optical communications, such low-cost integrated electro-optical devices and circuits can also aid in data transmission between electronic circuits or subcircuits or within an electronic integrated circuit. A greater percentage of optical signals used in such devices would help to reduce electromagnetic radiation or interference and decrease transmission speed within each device. This could lead to improved performance for high-speed electronic computers as well.
A future goal in optical communications systems is to replace part of the system with all-optical devices or circuits, which would enable faster operation. Such all-optical devices or circuits would also lead to the realization of ultrafast all-optical computers. Thus, devices that are capable of electro-optical operations or a mixture of electro-optical and all-optical operations would be very desirable.
Because optical pulses can be very short (in the femtosecond range), it is often suggested that all-optical switching can be very fast. There have been attempts to construct switches that partially use light beams to switch light beams in an attempt to increase speed. In such attempts, switching an optical beam with another optical beam typically involves electronics to translate an optical signal at some point to an electrical signal which is then returned back to an optical signal at a subsequent time. Optical communications systems based on such switches are not xe2x80x9call-optical communicationsxe2x80x9d because of this interface with electronic componentry. All-optical communications that allow the switching of light with light without the involvement of electronics as an intermediate step would reduce or eliminate the complexities inherent in the inclusion of electronic elements.
Below, examples of current art relating to all-optical switches as well as electro-optical switches are described.
There have been various attempts to switch light with light without the use of electronics. A typical method of switching one light beam via another light beam utilizes a Mach-Zehnder interferometer with a nonlinear optical medium. This implementation may be referred to as a nonlinear optical Mach-Zehnder interferometer. An exemplary Mach-Zehnder Interferometer 100 is illustrated in FIG. 1. The nonlinear optical Mach-Zehnder Interferometer 100 of FIG. 1 includes a pair of mirrors M1102, M2104 and a pair of 50 percent beam splitters BS1106, BS2108. A Signal Beam Input 110 input into the Interferometer 100 is split into a pair of beams 112, 114 via the 50 percent beam splitter BS1106. The beams 112 and 114 are recombined at the beam splitter BS2108 to form a pair of resultant beams. If the beams 112 and 114 face equal optical path lengths as the beams 112 and 114 traverse the upper and lower arms, respectively, of the Interferometer 100, then the beams 112 and 114 will constructively interfere to become Signal Beam Output A 116 and destructively interfere to become Signal Beam Output B 118. Hence, in this event, no signal beam will be output as beam 118 (as the destructive interference cancels the power at Signal Beam Output B), while the full combined signal beam will be output as beam 116.
A Nonlinear Refractive Index Medium 120 of length Lm, known to those skilled in the art as an optical Kerr medium, is positioned in the upper arm of the Mach-Zehnder Interferometer 100, as shown in FIG. 1. A Control Beam Input 122 with a polarization orthogonal to that of the beam 112 is introduced via a polarization beam splitter PBS1124. The Control Beam Input 122 propagates through and exits the medium 120 and is output from the Interferometer 100 via a polarization beam splitter PBS2126. The medium 120 has nonlinear optical properties, in that exposing the medium 120 to a strong light beam (in this case the Control Beam Input 122), can alter the refractive index of the medium 120. When the Control Beam Input 122 is on, the refractive index of the medium 120 will change according to the optical intensity, which is proportional to photons per unit time per unit area, of the beam 122. The refractive index of medium 120 can increase or decrease, which in turn causes the beam 112 in the upper arm of the Interferometer 100 to experience a change in the optical path length and to undergo an additional phase shift. This phase shift causes the destructive interference of the beams 112 and 114 at the beam splitter BS2108 to become constructive in forming Signal Beam Output B 118. Similarly, the phase shift causes the constructive interference of the beams 112 and 114 at BS2108 to become destructive in forming the Signal Beam Output A 116. This phenomenon leads to a net switching of signal output from the beam 116 (A) to the beam 118 (B). When the Control Beam Input 122 is viewed as a second input signal to the Interferometer 100, this dual input, dual-output all-optical switch can be viewed as performing optical logic operation equivalent to an xe2x80x9cANDxe2x80x9d gate used in the electronics realm.
The nonlinear optical Mach-Zehnder devices such as the interferometer 100 can achieve all-optical switching, but due to the lack of materials with a sufficiently high nonlinear refractive index, switches of this variety typically suffer from a number of problems and drawbacks. First, the device size (indicated by Lm in FIG. 1) is large. For a medium with a reasonably high nonlinear refractive index, a device length of 1 centimeter (1 cm) or longer is required. The large size of the device clearly prohibits their use in large-scale optical logic circuit integration. Second, the switching power required is very high, in that a control power of hundreds of Watts or more is required to operate the device at high speed. Third, while the nonlinear effect can be substantially higher when operated at close to the atomic resonance of the medium, thereby reducing the switching power, the speed of the switching operation will be slow due to real carrier excitation in the medium limiting the switching speed to below the hundreds of megahertz for a semiconductor medium. Fourth, the Mach-Zehnder device is very sensitive to device design parameter variations and vibration because of the dependence of the device on the optical path-length balance between the two arms of the interferometer.
Other variations of all-optical switching devices exist, such as one device (not shown) that uses a cavity to enhance the intensity in a medium or to achieve optical bi-stability. This device also suffers from one or more of the problems and/or drawbacks listed above with regard to the Mach-Zehnder device. These problems make the current all-optical switching devices impractical for applications to form large-scale or dense optical logic circuits. In fact it is often quite challenging to cascade even a few of the current all-optical switching devices to work together.
The nonlinear Mach-Zehnder interferometer described above and illustrated in FIG. 1 can also be implemented in a way to achieve electro-optical switching. Such an electro-optical Mach-Zehnder interferometer has the same configuration as the nonlinear-optical Mach-Zehnder interferometer except that the Kerr Medium of the Nonlinear Refractive Index Medium 120 is replaced by an electro-optical medium known to those skilled in the art as Pockel medium. The refractive index of the electro-optical medium can be altered via an applied electric field. The change in the refractive index leads to a change in the optical path length in one arm of the Mach-Zehnder interferometer, which again leads the Signal Beam Input 110 to exit the Signal Beam Output B 118 when the field is applied and to exit Signal Beam Output A 116 when the field is turned off, thereby achieving electro-optical switching. The electro-optical Mach-Zehnder switch may operate within a speed range of MegaHertz to tens of GigaHertz if the nonlinear optical effect is based on an intrinsically fast physical phenomenon such as the distortion of electron clouds around the atoms. Such distortion leads to a change in the microscopic electric dipole strength and in turn the macroscopic refractive index of the medium. One such medium commonly used is Lithium Niobate crystal. However, due to the smallness of such a fast electro-optic effect, a large electric field strength is needed to bring about complete switching. In typical devices, this translates into an applied voltage in the tens of volts, a high voltage value for high-frequency electronics.
In addition, these devices are typically large in size, with dimensions of several centimeters. A long interaction length is needed to attain the 180 degree (or xcfx80 radian) phase shift required for complete switching of the optical beam, due to the smallness of the electro-optic effect.
In order for computers to perform faster, and to circumvent many of the complexities that accompany electronic transistor computing at increased speeds, new compact technology must be developed. It would be advantageous to provide all-optical logic circuitry or a device family capable of improved speed, implementation at high or very high density of integration (due to smaller device sizes), operation at lower switching energy and power consumption levels, and improved immunity to device variations.
In accordance with the present invention, many of the disadvantages associated with prior electrical transistor-based devices and optical beam switching devices are addressed.
In particular, the devices described herein are very small in size with typical device dimensions of a few micrometers to hundreds of nanometers and are capable of achieving optical switching and electro-optical switching at high speed and low switching energy. Using these xe2x80x9cphotonicxe2x80x9d devices, which typically operate by manipulating photon flux, it is possible to build low-cost electro-optical optical communication systems, all-optical communication systems or all-optical logic gates that can operate at speeds of 10 GHz to 10000 GHz or faster. Such devices will enable the realization of computers operating with a clock rates from 10 to more than 100 times faster than that of current high-speed electronic computers. Furthermore, the compact size of the devices will allow very-high-density device integration (a few million devices per square centimeters), leading to low cost per device function and subsequently the enablement of complex operation.
In addition to their superior speed of operation, these devices are also advantageous in that the signal transfer in the photonic circuit is via an optical beam well-confined within optical waveguides. This configuration results in very little signal interference from surrounding componentry. Furthermore, optical beams do not radiate radio frequencies to cause such interference.
Like electronic transistors, these devices will have a wide range of other general applications apart from applications to computers, such as applications relating to optical communications, optical signal processing, optical sensing or quantum optical communications. For example, in optical communications, in addition to their low cost and applications to electro-optical switching, these devices will have important applications to the realization of ultrafast (Tera-bit) all-optical communications for which a stream of optical pulses is being switched at very high (Tera-Hertz) speeds directly via another stream of optical pulses without the complication and speed compromise involved in using electronics to transfer the optical signal to an electrical signal and back again.
Unlike all the current electro-optical and all-optical switches, such as that of the Mach Zhender type described above, where the switching of light signals is activated by changing the refractive index of the active medium, the devices of the present invention enable the switching of light signals via changing the photon resistance brought about by modifying primarily the absorptive, transparency, or gain property of the active medium. It will become obvious from the exemplary devices described below that the switching of light signal based on such xe2x80x9ctransfer photon resistancexe2x80x9d has many advantages over the current devices based on changing the refractive index of the medium. In particular, the devices of the current invention can be much smaller in size (10-10,000 times smaller), in operating power (10-1000,000 times smaller), and yet still relatively fast in switching time (hundreds of picosecond or faster). Such use of transfer photon resistance makes the devices of the current invention a close photonic analogue of electronic transistors. Hence, we call the devices of the current invention xe2x80x9cphosistorxe2x80x9d, which is short for xe2x80x9cphoton transistorsxe2x80x9d.
In one aspect of the present invention, a light transfer device is provided including a first light pathway having a first input and a first output and a second light pathway having a second output. The second light pathway is coupled to the first light pathway so that light from the first input is transferable between the first and second light pathways. An active medium is positioned along at least one of the first and second light pathways, and is capable of receiving optical energy that modifies the active medium so that the active medium controls the transfer of light between the first and second pathways.
In another aspect of the present invention, a light transfer device is provided including a first light pathway having a first input and a first output and a second light pathway having a second output. The second light pathway is coupled to the first light pathway so that light from the first input is transferable between the first and second light pathways. An active medium is positioned along at least one of the first and second light pathways, and is capable of receiving electrical energy that modifies the active medium so that the active medium controls the transfer of light between the first and second pathways.
In another aspect of the invention, a light transfer device is provided that includes a first light pathway having a first input and a first output and a second light pathway having a second output. The second light pathway is coupled to the first light pathway, and light from the first input is transferable between the first and second light pathways. A third light pathway is interposed between the first and second light pathways. An active medium is positioned along at least one of the first, second and third light pathways, wherein the active medium is capable of receiving light that modifies the active medium so that the active medium controls the transfer of light between the first, second and third pathways.
In yet another aspect of the invention, a light transfer component is provided having a first light pathway having a first input and a first output. A second light pathway is provided having a second output, and the second light pathway is coupled to the first light pathway. Light from the first input is transferable from the first input of the first light pathway to the second output of the second light pathway. An active medium is positioned along the first light pathway. The active medium is capable of receiving electrical energy that modifies the active medium so that a substantial portion of the light from the first input no longer remains on the first light pathway.
The invention may be further embodied in a logic device utilizing photonic energy. In this aspect, a plurality of directional couplers are provided, wherein the directional couplers each are capable of modifying the propagation direction of a photon beam through an active medium. The active medium has light transfer control properties that are modifiable via electrical energy. The directional couplers are linked by inputs and outputs capable of transmitting the photon beam through one or more switchable propagation paths.
In yet another aspect of the invention, a light transfer device is provided having a first light pathway including a first input and a first output, a second light pathway having a second output, and a third light pathway spaced from the first light pathway and the second light pathway. The second light pathway is spaced from the first light pathway, and the third light pathway is movable between at least a first position relatively near both of the first and second light pathways and a second position relatively far from the first and second light pathways. The first position allows the first light pathway to be optically coupled with the third light pathway, and simultaneously allows the third light pathway to be optically coupled with the second light pathway.
The present invention may also be embodied in a method of manufacturing a light transfer device. The method includes the steps of etching at least two waveguide structures onto a substrate material. The waveguides each include input and output ends, and at least one of the waveguides defines a gap area between its input and output ends. An active medium is then integrated onto the substrate into the gap area using epitaxial layer growth techniques to define an active medium section of the one of the waveguides. The active medium section preferably includes a plurality of quantum wells.
In another aspect of the present invention, a method of manufacturing a waveguide structure having at least two waveguides is provided. The method includes the steps of providing a silicon dioxide substrate, bonding a layer of GaAs to the substrate, patterning a photoresist for the waveguide structures, etching the GaAs not covered by the photoresist, and etching the silicon dioxide structure so as to form an undercut.
In addition to their use as optical switches, the disclosed invention can relate to the performance of a variety of functions, including logical operations, optical flip-flops, optical wavelength translation, electro-optical switches, optical detection, optical filtering, optical attenuation or gain, optical phase shifting, optical memory, and quantum optical operations, which take advantage of the physical effect of transfer photon resistance.
Beside their use as optical switches, the disclosed devices according to the present invention can perform a variety of multiple all-optical and electro-optical functions, including wavelength selective switching or filtering, variable optical attenuation/amplification or phase shifting, optical wavelength translation, optical diode, optical detection, optical memory, quantum-optical operations, optical flip-flops, and all-optical or electro-optical logic operations, all based on the physical effect of transfer photon resistance, which therefore creates a new class of devices. Potential applications of these devices include the realization of very-high-density photonic integrated circuits or nanoscale photonic (nanophotonic) devices and circuits, optical communications, optical sensing, optical interconnects, optical signal processing, all-optical computing, and all-optical communications.
The foregoing and other features and advantages of the presently preferred embodiments of the invention will be more readily apparent from the following detailed description, which proceeds with references to the accompanying drawings.